DESCRIPTION: The hypothesis for this project is that plasma factors, including Fas ligand (FasL)-related molecules, IL-l, and tumor necrosis factor (TNF)-alpha, play an etiologic role in HUS through induction of microvascular endothelial cell (EC) apoptosis. This is based upon three observations that we have made: plasma from patients with acute HUS induces apoptosis in microvascular EC of the kidney and certain other lineages, but not in large vessel or pulmonary microvascular EC; IL-l or TNF-alpha or both are up-regulated in subsets of HUS, reaching concentrations associated with apoptosis of EC in vitro; and acute HUS plasma induces the apoptosis-linked molecule Fas on subsets of microvascular EC, paralleling nuclear translocation of protein phosphokinases implicated in final common pathways to apoptosis. This hypothesis is consistent with the pathology of HUS, as EC activation, and detachment occur in the absence of an inflammatory response (one hallmark of apoptosis). We will examine the immunobiology of renal and other primary human microvascular EC in terms of interactions with HUS plasma, fractions of normal plasma, Fas, and a new member of the FasL family, which we have found on subsets of microvascular EC negative for FasL. Specificity will be sought using large vessel EC and pulmonary microvascular EC, which are not affected in HUS, and compared with the effect of plasmas from subsets of patients with a related thrombopathy, thrombotic thrombocytopenic purpura (TTP), who show limited renal pathology. In vivo evidence for apoptosis will be sought using HUS autopsy and biopsy material. Based upon the premise that Fas/FasL-like interactions are facilitated by HUS plasma factors, leading to microvascular EC activation, protein kinase induction, and EC apoptosis, we will examine the possibility of developing treatments involving inhibitors of apoptosis.